Generally, a selective catalytic reduction (SCR) system is a system which reduces nitrogen oxide by purifying exhaust gas which is generated in a diesel engine, a boiler, an incinerator, or the like.
A selective catalytic reduction system reduces nitrogen oxide into nitrogen and water vapor by reacting nitrogen oxide contained in exhaust gas with a reducing agent while passing the exhaust gas and the reducing agent through a reactor in which a catalyst is installed.
A selective catalytic reduction system uses urea directly injected or ammonia NH3 generated by hydrolyzing urea and then injected, as a reducing agent adapted to reduce nitrogen oxide.
Generally, a sensor is installed in front of a reactor for selective catalytic reduction in which a catalyst is installed, a nitrogen oxide concentration is actually measured, and an appropriate amount of reducing agent required to reduce nitrogen oxide contained in the exhaust gas is injected.
However, reaction efficiency varies depending on various factors, such as the degree of activation of a catalyst, whether a catalyst has been poisoned or not, the temperature of exhaust gas, etc. When the concentration of nitrogen oxide is actually measured before a selective catalytic reduction reaction, the amount of reducing agent is quantitatively determined, and then a corresponding amount of reducing agent is injected, various factors and errors occurring in an actual reaction are not taken into account, and thus a problem arises in that it is difficult to precisely control a reduction in nitrogen oxide contained in exhaust gas.
Furthermore, in the future, such a selective catalytic reduction system is required to have performance and operation in which the amount of nitrogen oxide (NOx) emitted from a diesel engine for a ship meets the International Maritime Organization (IMO) Tier-III requirements.
Meanwhile, when ammonia which is used as a reducing agent is excessively injected in order to sufficiently meet the International Maritime Organization (IMO) Tier-III requirements, ammonia slip occurs. Ammonia slip is a phenomenon in which unreacted ammonia which did not participate in a reaction is emitted to the outside along with exhaust gas when an amount of ammonia larger than the amount of ammonia which can quantitatively react with nitrogen oxide is injected.
When ammonia slip occurs, unreacted ammonia may reduce the life span of a catalyst or may cause the corrosion of the catalyst. When unreacted ammonia is emitted to the outside along with exhaust gas, an air pollution problem attributable to ammonia may occur additionally.
In contrast, when the amount of ammonia which is used as a reducing agent is insufficient, nitrogen oxide contained in exhaust gas cannot be sufficiently reduced, and thus the IMO Tier-III requirements are not met.
Accordingly, there is required the operation of a selective catalytic reduction system which can meet the IMO Tier-III requirements not only in the initial stage of the operation of a diesel engine but also in the case of a variation in load and which can also minimize the occurrence of ammonia slip.
Accordingly, in order to control the amount of reducing agent to be supplied so that an appropriate amount of reducing agent can be used, conventionally, the amount of nitrogen oxide to be removed is calculated using the total amount of exhaust gas, calculated by using the amount of intake air measured using an MAF (mass air flow) sensor and the amount of fuel of an engine calculated using an electronic control unit (ECU), and a nitrogen oxide concentration measured in exhaust gas, and then the amount of reducing agent to be supplied is controlled based on the calculated amount of nitrogen oxide to be removed.
However, in the case of an engine used for a ship, a problem occurs in that it is difficult to ensure the durability of an MAF (mass air flow) sensor due to salt, moisture, or the like.
As a result, for environmental or economic reasons, there is a need for a method capable of controlling the amount of reducing agent to be supplied without requiring an MAF (mass air flow) sensor.